Marine and meteoric diagenesis of submarine pleistocene carbonates from the Bermuda Carbonate Platform
Marine and meteoric diagenesis of submarine pleistocene carbonates from the Bermuda Carbonate Platform
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百慕大碳酸盐台地海底更新世碳酸盐的海洋和大气成岩作用
DOI：
10.1007/bf03174319
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发表时间：
1990
影响因子：
1.4
通讯作者：
R. Vollbrecht
中科院分区：
文献类型：
--
作者：
R. Vollbrecht
构造稳定的百慕大碳酸盐岩台地在更新世海平面变化过程中经历了反复的出露和淹没。浅层地震反射剖面分析和高能振动取芯使人们得以进入目前位于海平面以下的更新世碳酸盐岩。进行了成岩作用的研究，以记录在各种成岩环境中发生的变化，检查成岩保存状态和地层年龄之间可能存在的相关性，并估计沉积物孔隙水历史的完整成岩成像的可能性。为了进行对比，还分析了现代百慕大群岛的几个样品，从生物成分来看，海底更新世碳酸盐岩在成岩作用开始时主要由文石和高镁方解石组成，而现在则普遍呈现为文石和低镁方解石的混合物。海相胶结物（5种高镁方解石胶结物、3种文石胶结物）一般局限于颗粒内孔隙。大气影响使海相碳酸盐岩发生新生变质作用、低镁方解石胶结作用（几种胶结物）和溶解作用。其特征是斑片状的岩化作用、丰富的保存孔隙空间和不完全的矿物稳定作用。此外，特别是在泻湖沉积物中，洞穴或根部结构优先被胶结。岩溶和钙化的指标是常见的。高镁方解石通常是第一个矿物消失在流星的影响下，文石更稳定。强烈蚀变的碳酸盐岩完全由低镁方解石组成，而白云石则根本不存在。大气降水潜水蚀变的成岩指示通常不常见，但支持淡水透镜体的想法，这些透镜体在很大程度上局限于目前的岛屿附近。然而，大多数碳酸盐岩在大气渗流环境中获得了决定性的成岩印记，其内部的成岩记录是不完整的。只有一个孔隙水的变化记录，从海洋到大气环境。毫无疑问，在所研究的更新世物质中，除了一些强烈的岩化沿岸的沉积物外，没有发现后陨星海相胶结物。显然，海洋环境的成岩潜力太小，无法与大气环境竞争。在这种情况下，根据成岩序列重建孔隙水历史是具有欺骗性的。
The tectonically stable Bermuda Carbonate Platform has undergone repeated emersions and submersions in the course of the Pleistocene sea-level changes. Shallow seismic reflection profiling and high-energy vibration coring gave access to Pleistocene carbonates that are presently situated below sea level. A study of their diagenesis was undertaken in order to document the changes occuring in various diagenetic environments, to check a possible correlation between the state of diagenetic preservation and stratigraphic age, and to estimate the odds for a complete diagenetic imaging of a sediment’s pore-water history. For the purpose of comparison, several samples from the modern Bermuda Islands were analysed as well.As indicated by their biogenic constituents, the submarine Pleistocene carbonates were chiefly composed of aragonite and high-Mg-calcite at the onset of diagenesis, whereas they now commonly appear as mixtures of aragonite and low-Mg-calcite. Marine cements (five kinds of high-Mg-calcite cement, three kinds of aragonite cement) are, as a rule, restricted to the intraparticle pore space. Meteoric influence inflicted neomorphism, cementation with low-Mg-calcite (several kinds of cement) and solution on the marine carbonates. These are characterized by patchy lithification, abundant preserved pore space and incomplete mineralogical stabilization. Furthermore, especially in lagoonal sediments, burrow or root structures have been preferentially cemented. Indicators of karst and calichification are common. High-Mg-calcite is usually the first mineral to disappear under meteoric influence; aragonite is more stable. Intensely altered carbonates consist entirely of low-Mg-calcite, whereas dolomite does not occur at all.Diagenetic indications of meteoric-phreatic alteration are generally uncommon, but support the idea of fresh-water lenses that are largely restricted to the proximity of the present islands. Most of the carbonates, however, have received their decisive diagenetic imprints in the meteoricvadose environment.The diagenetic record within the carbonates is incomplete. Only one pore-water change is recorded, from a marine to a meteoric environment. Undoubted post-meteoric marine cements are absent from the studied Pleistocene material except for some intensely lithified littoral deposits on the present islands. Obviously, the diagenetic potential of the marine environment was too small to compete with the meteoric environment. Under these circumstances, a reconstruction of pore-water history based on diagenetic sequences is deceptive.